Does dark matter have structure?

We put this question to Dr Andrew Pontzen:

Andrew - Well firstly, normal matter in the universe is really clumped together. It's not evenly spread out through the universe, it's clumped, for instance into galaxies. Galaxies are collections of around 100 billion stars, and they're relatively compact, and there's a lot of relatively empty space between each different galaxy. Now, dark matter certainly does clump together in that sense. We know that for sure, from observations of galaxies.

This was in fact the original evidence for dark matter, looking at the way that material like stars and gases moves around in galaxies, and inferring from that strength of the gravitational field in galaxies, and from that inferring how much stuff was there, and that's how we knew that there had to be dark matter.

In fact, because there's so much more dark matter than normal matter in the universe - there's around five times more dark matter than normal directly visible matter - its clumping is incredibly important in terms of determining the kind of structures that form in the visible universe, and the existence of galaxies effectively owes itself to dark matter. So in that sense, there are structures in the dark matter that are similar to the ones you see directly in the normal matter.

On another level though, we don't really know what dark matter is, and so, when we're talking about dark matter, we tend to be modelling it subject to some simple assumptions about what it's doing, and you get the best results for the evolution of the universe, matching what we see in the real universe, when you model the dark matter as completely non-interacting, except through gravity. So, other than the gravitational force which it exerts, and which it's also subject to, it's not subject to any other forces, for instance, the electromagnetic force which normal matter is subject to. In fact, the really interesting structures in normal matter arise through things like the electromagnetic force in all of chemistry for instance, and therefore, life really arises through forces like the electromagnetic force. And for that reason, the evidence at the moment would suggest that you can't have really complicated structures that will be required to create what you might describe as dark life forms. So, most likely, there's nothing quite that interesting going on in the dark sector, but until we really know what it is, we can't say for absolute definite.